They sell rocks with painted faces, glue on eyeballs, and give them names. Yes you cam sell anything. Someone even bought a piece of left over food from someone else's thanksgiving dinner on e-bay for a outrages amount of money. You create it, someone will buy it (provided you make it sound as if they need it). Fact is you don't need a PPM,TDS, or EC meter to grow plants hydroponically. Though you probably wont hear that from a hydroponic supply retail store, because they want to sell you stuff. First you should understand the capability's of the meters to understand why.

All of these meters do basically the same exact thing (just using different measurements). All of them use electricity to find out the electrical conductivity (EC) of a liquid, and give you a number from that reading. The nutrient solution is made up of many mineral elements, and not one of these meters can tell you what your nutrient solution is made up of. You will often hear people speak of there water supply having a certain PPM to start with. That is because unless it's distilled water it will have some elements in it (even if it is filtered). Again none of these meters cant tell you what's in it, or in what quantity each element is in. Not even if they are desirable elements or harmful ones.

If you want to decide when to change the nutrient solution simply by the EC/TDS/PPM readings. The answer is simple, you never need to change the nutrient solution. All you need to do is adjust the readings by adding more nutrients to raise the numbers, or just add plane water until it's diluted to the range you want. Doing that you can keep the nutrient solution within a specific range indefinitely (even 20, 30 years if you wish).

But here's the problem, as I mentioned not one of these meters can tell you what's actually in the liquid, and/or how much of each there is. For the nutrient solution to be beneficial to the plants, all the mineral elements within it need to be somewhat in balance. But plants don't absorb the all the elements within the water in a balanced fashion, they use only what they need, and leave the rest behind. This creates an unbalanced solution. The farther unbalanced it gets, the more the elements the plants are using in larger quantity become depleted, and faster than the other elements (thus unbalanced). Because some elements were used, that will drop the EC/TDS/PPM readings.

The simple fix is to add more nutrients and bring up the readings to the acceptable levels again. This would work except that as I mentioned, the elements the plants don't use were left behind, and by adding more (balanced) nutrients to the solution, your also replacing more of the elements that the plants didn't use in the first place. So you still wind up with an unbalanced solution (even when your meter readings are in the correct range). Too much of this leads to the elements the plant isn't using building up in the nutrient solution, to a point they reach toxic levels. Your meter is just telling you the total amount of elements in the water. So you wind up with toxic levels of some elements, and deficiency of others, but together they add up to the right number of elements in the water. The meters simply cant tell you PPM level of:

There's just no way for the meters to tell you which elements that are lacking, or which are in excess. If you need 3 oranges, 3 grapes, 3 lemons and 3 limes, for a total of 12 to make your recipe work. It won't be the same with 6 oranges and 6 lemons for the required count of 12. Even if they could tell you which specific elements to add, you still would need to have individual mineral elements in order to replace just what's lacking. Using Pre manufactured hydroponic nutrients (that contain all elements in a balanced recipe), that's not possible.

The reason you cant find charts, or step by step how to know, or an understandable formula on when to change the nutrient solution, is because they don't exist (for the reasons I mentioned). There is just no way to tell what is actually in the water (nutrient solution), or the levels of them based on EC/TDS/PPM readings.

The best (creditable info) you'll find on changing the nutrient solution or PPM levels will be based on specific plants, and related to the growth stage the plants are in. Even so, there are still many environmental conditions (light, temp, humidity, etc.) that will affect how your plants absorb the individual elements in the nutrient solution. If anything they will be just a guideline, and assuming your nutrient solution is within a balanced range. In fact any chart that you may find should have the disclaimer that they are assuming the nutrients in the solution are balanced at all times. Which is imposable to even guess, unless you change them regularly (or have a lot of money to have a lab test it).

Think of a EC/TDS/PPM meter as just a tool that's a little more specific in telling you the relative strength of your nutrient solution (relative because it cant tell you specific levels of each mineral element) than would be if you are fallowing the manufactures mixing directions. But it doesn't matter what the strength of the nutrient solution is if it's not balanced. Anymore than it matters that the nutrient solution "IS" balanced if the pH is out of range.

There are many rules of thumb on when to change the nutrient solution, and every guarder eventually figures out what works best for them. Everywhere from once a week, to once or twice a year. Things like the size of the reservoir, temp, size of the plants, the growth stage the plants are in, etc. all affect their decisions. Personally I change mine somewhere between once a week and once a month, again depending on the variables (including abnormal pH swings). But I rely mostly on observing the plants themselves. I look for singes of discoloration, and growth slowing down (that isn't related to environmental conditions like light and temp). Bottom line, I rely on observation to let me know when to change my nutrient solution. I don't even own a EC/TDS or PPM meter, I have a lot more useful things to spend $100 on.

If you are not sure and want a guideline, start by changing them once a week for a month, then go to every other week, then stretch it out to three weeks the next month (and so on). All along observe the plants, look for signs of yellowing (dark green leaves turning to light green), growth slowing down, rapid or frequent pH swings. As long as you already have it use the meter to keep the relative strength within specific range, that will also aid in the accuracy of the experiment. Within a few months you'll be able to predict when to change the nutrients before you need to. Once you get a hang of that, try using stronger and/or weaker nutrient solution strengths, and again observe the results.

"Do different plants have different levels of ppm they respond to and react to?"

I don't usually post this, but this page/s was created for that very question. Keep in mind that EC/TDS/PPM is just the relative strength of the nutrient solution, the numbers really only refer to a stronger or weaker nutrient selution. And as always it also assumes that your nutrient solution is always balanced. ph/cF/EC/PPM Levels for Fruit

Thanks homehydro.
This is kind of what we thought.
The professional growers use the meters after a lot of experience with whats going on with their systems and all the conditions involved.
We have discovered that aeroponic nutrient solutions PPM stays lower than our flood & drain systems in the same length of time. Maybe wrongly but have come to believe the hydroton adds to the ppm as we have found no way to reach a clean rinse between uses or even with new hydroton. There is color in the rinse, we just give up after so many rinses and use it right or wrong.
Something tells me you understand what I'm trying to say here. Again thanks for the reply.
"Doing 10 with no Chance of Parole"

EC is the only thing measured. Therefore, it is the only thing you can rely on. cF works, too, as it is just EC x 10. PPM and TDS are totally inaccurate on handheld meters as they are a conversion of a number that has absolutely nothing whatsoever to do with PPM or TDS.
You asked:

"Do different plants have different levels of ppm they respond to and react to?"

Yes. As HH said, you will have different values for different stages of growth. However, you can usually find charts that give a range. Below is one. Notice the term cF over one column. Use it. It's EC x 10.

HH said:

"If you want to decide when to change the nutrient solution simply by the EC/TDS/PPM readings. The answer is simple, you never need to change the nutrient solution."

Not sure why he said that. Especially since he also listed good reasons to change solution and even said he changes his own once a week to a month. Yes, you should change the solution regularly. Is it necessary to get results? No. Is it necessary to maximize results and reduce risk? Absolutely. If you go commercial, those issues can make or break you.

HH said:
"There is just no way to tell what is actually in the water (nutrient solution), or the levels of them based on EC/TDS/PPM readings."

That is actually the second best reason to change solution regularly. Not all nutrients have the same conductivity. In other words, you can have equal amounts of two minerals in two different reservoirs filled with water. Take an EC reading and you may find two totally different results. So if you get low on solution or get a low EC reading, you can add nutrient solution. Usually people do this every few days (top off). It depends on the system you have and how much water is used up in a certain amount of time. After 3 weeks of this, if one mineral keeps getting used up faster and another, lets say calcium, isn't used up in near the same proportion, you end up with a rez with high calcium content. This will throw off your subsequent EC readings and tell you that you have more nutrient in the solution later than you really have. It will also cause huge pH swings as your ratios end up way off. You should change solution regularly to avoid the problems and causing your plants to waste huge energy and time on recovery that could easily be prevented. That impacts yield big time. It you are making a living off this, a week late on harvest or hundreds of pounds light on harvest could cost you a ton more than the nutrients you used to be on the safe side did.

Another reason, related to the first, is that an imbalance can lead to fallout. You will start to notice build up in small emitters of drip systems and spray systems. That causes you more labor and potentially more replacement costs. From a business standpoint, your labor time is money. The labor saved from making sure your solution is ideal compared to money saved from not replacing solution needs to be factored in along with the risks I listed above and also with the next risk, which is the biggest reason to change solution, in my opinion.

So on that reason. . . Contamination. After a few weeks, your risk of contamination by biological agents is increased quite a bit. The water isn't sterile. It will grow bacteria and fungi. It's not that big of a deal if you have small growth in a fresh system, but if it's had weeks to populate your reservoirs, you end up with something the plants can't cope with. You spend time and money on fixing the problem and the plants lose yield and time during recovery. Worst case, you could lose all plants. All to avoid a few dollars and a few minutes on prevention. How senseless is that? Why wait until you have the problem that can cause such losses when you can easily reduce risk significantly? I'm a firm believer that many of the problems claimed on forums could have been prevented had people taken preventative measures that many claim are unnecessary.

"The answer is simple, you never need to change the nutrient solution."

To the basic question on when to change your nutrient solution based on the PPM readings. Well cant you just add nutrients if the reading is low, and/or dilute it if it's too high? Yes, therefor if you just want the meter readings be in a specific range, it isn't necessary to change the nutrient solution at all to get the readings within those measurements. That's why the specific statement.

However You missed the part where I said "(even 20, 30 years if you wish)." This means that it's possible to keep the nutrient solution within the specific range as long as you wish indefinitely, simply by adjusting the nutrient levels in it. Therefore, if you wanted the PPM readings to dictate when you change your solution, you would never need to. Most people would understand that using the same nutrient solution for decades (even when it's within the specific PPM range wanted) would not work (as an extreme example). That was intended to get the hamster on the brain wheel moving, and wonder why.

That led the way for me to go into the reasons NOT to rely on EC/TDS/PPM readings to decide when to change your nutrient solution (as you pointed out that I did). That was the entire point.

OK Joe & hydro you guys know this stuff, that is obvious.
I understood both of you quit well. Both of you are easy to understand and well versed.
The charts are great, really don't understand why they never came up when we googled.
BUT, I don't understand the chart on PPM. For instance the chart for my strawberries is 1260-1540. Does that mean 1540 is the high level that the plants won't thrive? But do you have to be above 1260? That doesn't sound right, as the PPM in a fresh batch of nutrients will not be 1260 depending on what growth cycle we're on. Maybe someone could talk about what these levels mean to the plants, You know me hydro, I need the aeroponics for dummies version
As I said before the PPM in the flood & drain system rise faster than the aeroponic do. Is that a correct observation?
"Doing 10 with no Chance of Parole"

Soyousee
We all feel like we need the for dummies version of hydroponics/aeroponics at first. Don't worry, your normal. The PPM reading is again just a basic relative strength of the nutrient solution.

1260-1540 is a general range. General because it doesn't specify a specific plant variety (species of strawberry), or stage of growth (small plants, full grown, in-between, fruiting, heavy fruiting, etc.). The higher the number the more minerals that are in the water, thus the stronger the nutrient solution is.

1260 (is the suggested low end)
1540 (is the suggested high end)

You will probably find a lot of people who grow strawberries, and their PPM levels are outside this range (if they even check it), and they say there plants are doing great. They may very well be. That isn't to say if your PPM level is outside of this range, your plants won't grow or die. That it' isn't to say if they are within that range the plants wouldn't do better either. Remember the specific growing conditions (temp, light, type of nutrients used, humidity, co2 levels, dissolved oxygen levels in the water etc. etc. etc.) all will affect how the plants take up nutrients. Example: like I mentioned the range does not specify temperatures, and generally speaking in warm/hot conditions plants do better with a weaker nutrient solution. So results will always vary.

As for your observation about the PPM rising in your ebb&flow system, I'm a bit unclear as to the specifics. For one are you saying that you started with a PPM of say 1260, and a few days later that same system is now up to say 1540, for an increase of 280 PPM? Or are you just noticing that the PPM numbers are not dropping as fast in the
aeroponic system when you compare it to the flood and drain? Can you be specific as to the exact readings you are getting, as well as when, and which systems the readings are from?

If you are comparing the PPM measurements between two systems and thinking the Numbers should drop at an even rate. Again the growing conditions, type of plant, growth stage of the particular plants etc. etc. etc. will affect how the plants uptake the mineral elements. So there is bound to be a difference. Aeroponic systems also tend to produce more smaller hair like roots than a flood and drain system will. These small hair like roots absorb the mineral elements faster too. That's another difference in growing conditions.

If you are saying that in your flood and drain system starts out with a PPM of say 1260, but then in a couple days later the PPM is now say 1540 for a increase of 280. There could be a few reasons. First (and the most likely) is the water level. As the plants transpire (breath) they drink up water. The nutrients that are left in a smaller volume of water are more concentrated (stronger), thus a higher PPM will be exactly what would happen. I always mark the inside of my reservoirs, so I can top off the water level to where it started at. This keeps the nutrients from becoming too concentrated. Also the size of the reservoir will have a big effect on changes within the nutrient solution (as well as the size of the plants in the system).

Another possibility is something is somehow being introduced into your nutrient solution. I don't know if residue from the grow rocks would exactly do it, but that's certainly plausible. Just about any chemical or mineral will have some conductivity to it. Thus increase the electrical current that the meter records, and in turn raise the PPM readings it shows. If the raise in PPM was due the grow rock residue, I would expect to see it slowly go away as more and more got washed out, and then dumped with each nutrient change. Lastly is the care and maintenance of the meter itself, and/or if it's functioning properly could be an issue. Unfortunately it wouldn't be the first time someone experienced false readings from a meter, either from a faulty meter, and/or because of care and maintenance issues/habits.

I'm also not sure what you mean by your nutrient solution wont be above 1260 on a fresh batch. All you need to do is add more or less nutrient concentrate to the water to get the desired reading. If you mean that if you fallow the manufactures mixing directions for the specific growth stage, and for the particular plants. Your PPM readings don't seem to match up with the general chart PPM suggestions. That isn't unusual. Again the charts are just general guidelines (not specific). You are better off fallowing the nutrient manufactures instructions, again here you can use the meter to tell if the solution is stronger or weaker than their recommendations, as you continue to grow and care for your plants. In other words, as a reference tool to judge how your plants grow best.

But there will always be differences between PPM recommendations between nutrient manufactures. Also there will be differences in actual PPM/TDS readings between manufactures of the meters, and/or even through different areas of the world. The meters take a electrical current, and turn that into a number. With EC meters that will be consent. But with PPM and TDS meters, they rely on a conversion rate that varies from manufacturer to manufacturer, and area to area.

Nutrient manufactures just cant specify different PPM/TDS readings for different growth stages for every manufacture around the world. But they do know there own nutrients, and that's why you would be better off relying on their mixing directions. Nutrients generally contain all the same mineral elements, but each manufacture has their own recipes, for mixing them (the formula), and some are more plant specific than others. So PPM charts cant take into account all the variables here.

Nor will you see a chart that lists all the different PPM ranges, for all the different conversion rates. Some may specify the conversion rate they are using, but that's rare, and not likely to list many different plants. Even so, they would only be specific if they specified the particular nutrients they were using as well as the conversion rate used to get the PPM numbers.

In general
Seedlings--(about 1/4 strength nutrient solution)
A little bigger--(about 1/2 strength nutrient solution)
Young but becoming established to full grown--(about 3/4 to full strength nutrient solution)

I like to mix mine a little on the weaker side even when the plants are established and/or full grown (typically about 80%-90% strength, and about 70% in hot weather)

I didn't have time to read HH's post yet, so I might be repeating him. Some charts show the range based on normal vegetative vs. bloom/fruiting differences. You usually have higher EC for blooming/fruiting as you use a different formula. Some people really push it with an extra increase in nutes. Some rob the plant of nutes and then "shock" it with the higher EC bloom nutes to force blooming. I don't know if that really works, but I tend to at least flush between formula changes. I don't try to shock, though. I think it's more common for flower producers. Check out general hydroponics' nutrient calculator and you will see they show a significant range throughout the growing cycle.

Probably a never ending debate and still a bunch of residing misconceptions....

Since I've seen them, I have simply ignored these ppm charts. Why? Because there is no such thing as a diversified recommended ppm for several vegetables or types. Firstly, they are overruled by a few factors and variables, like choice of system, climate and Evapotranspiration (ET) and secondly, most importantly the type of nutrient and formula, which in fact results in a specific strength. It is indeed the other way round because a nutrient formula and strategy actually results in a certain strength.

In the same way and with the same knowledge I acquired over time, I did disapproved and actually rejected those "famous" Ph charts as well. These are simply transferred from soil culture and collected from multiple and not listed, actually non-scientific sources. They may have some personal value to some, even find partial consensus in some case or the other - but that's all I would attribute to them. Personally and using all the common sense I can gather, I am not interested in them, at all ;-)

As for the advantage and necessity of an EC-meter, I can only say that I prefer suffering from dyschromatopsia instead of complete sightlessness. I've btw. never been taken in by the idea that any EC-meter is capable of measuring and/or able of distinguishing between single elements of a nutrient solution. Like most of "us", I was confused with conversation rates and the contradiction of measured (converted) ppm of various standards and elemental ppm of formulas as well. Although this is part of a more advanced level of understanding, once you have got it and put into context, there is no more confusion or limitation here either.

From my understanding and daily practice, an EC meter is simply an essential and strictly necessary tool to measuring and determining nutrient strength as well as maintaining it at a certain level. It's certainly good to know all the limitations there are to it, (and not fall for any wrong conclusions or early anticipations) - but it's not relevant for the choice of using or not using them, as soon and as long as you are aware of those. All I want to know and measure is the nutrient strength of a fresh solution and to top it off as good as I can eventually - although being aware that it's composition has obviously changed. Mainly due to different uptake rates of elements and components.

Either you are fully aware and sussed with nutrients and individual uptake rates, even familiar with + and - charges in ion exchange, - or fall for any imaginable wrong assumptions about the topic - An EC-meter and PH-meter are in anyway the doctors stethoscope and blood pressure tool to the hydroponist. Both are actually essential and equally important. As well for the amateur as for the pro.

Anyone who has never owned nor used any EC-meter and simply wants to keep on going with simplified dosage, thinking in spoons per gallon and manufacturer's recommendations or even "half strength" (whatever that always means) - fine, go on with it and tell your personal opinion about it as extensively as you wish. But please do those who are actually interested in understanding EC-meters and related units, acquiring and dealing with them eventually just a little favour: Don't write a library of Amalia full of books about the limitations and the unnecessity of them, if you haven't ever used any. Thanks for that ;-)

Bottom line: despite their limitations, measuring conductivity in Simens per centimeter is what EC-meters (btw. best is to simply forget about ppm in this context) do perfectly well for me and for many folks.